1. Field of the Invention
This invention relates to the preparation of partial hydrolyzates of orthosilicic acid tetra(alkoxyalkyl) esters having stable shelf life. More especially, this invention relates to an improved process for the preparation of partial hydrolyzates of orthosilicic acid tetra(alkoxyalkyl) esters by hydrolysis of such orthosilicic acid tetra(alkoxyalkyl) esters with water wherein the stoichiometrically required amount of water is present at the commencement of the reaction.
2. Discussion of the Prior Art
The preparation of solutions of partial hydrolyzable silicic acid esters as well as their use as a binder for finely distributed compositions of matter is well known. Systems containing such hydrolyzates find use in the preparation of zinc powder coatings as well as in the technique of ceramic casting molds for precision casting or investment casting.
In most cases, the preparation of the partial hydrolyzates is derived from orthoethyl silicate or from polyethyl silicate, whereby both are easily obtained from silicon tetrachloride and ethanol. The silicates mentioned above can be produced by a simple process and, because of their relatively high content of SiO.sub.2 and their reactivity toward water, possess advantages relating to commercial application. Their reactivity to water as well as dilute acid during separation of the easily detachable ethanol, whereby the partially hydrolyzable and condensed silicic acid ester and finally the SiO.sub.2 capable of binding evolve, determines the commercial value of the product. By-product ethanol exhibits several drawbacks, such as its low flash-point and the low evaporation value, making these compositions in some areas of utilization unfit as binding agents. Because of these disadvantages, it became desirable to search for other starting silicates as silicic acid ester binders.
Besides the flash point and the volatility of the alcohols, the Si content of the compound, usually cited as SiO.sub.2 content, plays a deciding role in the selection of suitable silicates. For use as a binder, the SiO.sub.2 content of the composition should not be too low. The above-mentioned ethylsilicates, with their SiO.sub.2 content of approximately 28 to 40 weight percent, show enough downward range for the hydrolysis reaction with water and the addition of suitable solvents.
When changing over to silicic acid esters of alcohols containing 4 carbon atoms and whose properties correspond to the above-mentioned requirements, as for example, ethoxyethanol, the SiO.sub.2 content of these esters decreases to approximately 15 weight percent. If such an orthosilicic acid ester is hydrolyzed in the usual manner in the presence of corresponding solvents with equally higher flash-point, the SiO.sub.2 content of the resulting hydrolyzate is additionally lowered in inadmissable manner. In addition, when the added mixture of dilute acid and solvent is, too lean on solvent content, there results cloudy solutions from which a sediment gradually results. This occurrence seems to be caused by the fact that the gradually evolving hydrolysis products exhibit only low solubility in the unchanged starting ester and require a certain amount of solvent as a means for solubility, in this case especially ethoxyethanol.
In order to obtain a technically useful binding agent from ethoxyethyl silicate, the procedure of British Pat. No. 13 51 502 begins from a pre-condensed product with at least 19.1 percent SiO.sub.2. The above is hydrolyzed with dilute hydrochloric acid in the presence of enough suitable solvent so that the partially hydrolyzed silicate does not settle out of solution.
Binding agents mentioned in German Offenlegungsschrift No. 21 47 865 are also based on polysilicates with at least 19 percent SiO.sub.2. Such relatively high SiO.sub.2 content in silicic acid ethoxy alkyl esters is obtained only with polysilicates rich in SiO.sub.2 containing Si--O--Si-- combinations in addition to the ester groupings attached to the Si atom. Usually, these polysilicates are hydrolyzable with dilute acid to the desired degree of hydrolysis in the presence of suitable solvents. The objective of the present invention consists in simplifying this complicated, multi-stage procedure for the preparation of a solution of partially hydrolyzed ethoxyethyl silicate.